In the humid foothills of the Caucasus Mountains, deep within a carnivore's bloody lair, an early human ancestor fought a life-or-death struggle, and lost.

He had entered the den on a scavenging mission, possibly with several others. Their plan: Use a stone to scrape meat from the bones of freshly killed prey, then flee before a saber-tooth cat or other giant predator caught him in the act.

"It seems that they were fighting for the carcasses, and unfortunately ... they were not always successful," said David Lordkipanidze, a paleoanthropologist and director of the Georgian National Museum in Tbilisi.

Now, almost 2 million years later, the stunningly intact remains of that failed foraging mission are causing researchers to question the shape of our ancestral family tree.

Most notable among the fossilized relics are a cranium and jaw of an adult male that together comprise "Skull 5."

In a paper published Thursday in the journal Science, Lordkipanidze and colleagues say that skull and four other fossil craniums recovered at the site contain features previously ascribed to three different species of human ancestor: Homo habilis, Homo rudolfensis and Homo erectus.

The explanation for this is clear, they say: All three species must be one and the same. Differences once perceived to be the mark of separate species are in fact the result of normal variation in physical features, age and gender, they say.

The assertion has struck a nerve in a field where some paleoanthropologists complain that peers are all too quick to classify small or badly crushed fossil finds as evidence of new species.

"It's a little like the Emperor's New Clothes," study coauthor Christoph Zollikofer, a neurobiologist at the Anthropological Institute and Museum in Zurich, Switzerland, told reporters. "At some point you have to step out of a given perspective and take a new one."

Reaction to the paper has been strong.

"This is significant," said Tim White, a UC Berkeley paleoanthropologist who was not involved in the study. "I think that years, even decades from now, this will be seen as a classic turning point. It's not going to be received well by those who claim our family tree is more like a creosote bush than a saguaro cactus."

Others say that while the discovery of Skull 5 is a spectacular fossil find, the authors have failed to convince them that it applies to fossils recovered in Africa.

Fred Spoor, a professor of evolutionary anatomy at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, said he's not about to "say goodbye to Homo habilis and Homo rudolfensis."

Though the authors focused on the shape of the fossil craniums, that is not the sole indication of a species. Newly evolved features, however, were.

"Whether you're talking about hominids, ants or frogs, you don't make a species solely on the basis of overall cranium shape," Spoor said.

Ironically, Lordkipanidze and his team initially classified their fossil discoveries as a separate species — Homo Georgicus — in 2002. After further analysis, they retracted that classification and now describe the fossils as belonging to Homo erectus. All of the fossils were found in Dmanisi, Georgia.

The Dmanisi hominids are the oldest ever discovered outside Africa, and researchers say they were probably among the first members of Homo erectus to begin migrating throughout the Old World. Evidence of Homo erectus has been found from Spain to Indonesia to China.

It's unlikely the Dmanisi hominids had much in the way of travel plans when they first arrived in Eurasia, researchers said.

"People in Africa didn't just pack their suitcases ... and leave for Dmanisi," Zollikofer said. "It was dispersal. There was no aim."

Each of the recently found skulls is notable for its large face, heavy brow and protruding jaw, as well as a very small brain case — about one-third the size of modern humans'.